1. Field of the Invention
The present invention relates to methods of preparing products by freeze-drying.
2. Description of the Prior Art
Freeze-drying is a well known method of drying heat-sensitive materials in order to protect them from thermal damage. In the past, preparations containing active ingredients, such as pharmaceuticals, nutrients, diagnostics, fertilizers and insecticides, have been prepared by freeze-drying aqueous solutions or suspensions containing these active ingredients. One problem that has arisen, however, with the use of known freeze-drying processes is cracking of the freeze-dried preparations. Typically, cracking is caused by the stresses set up during ice crystallization. Though cracking is never desirable, it is especially undesirable where drop methods of freezing are employed. In such cases, cracking of the frozen droplets usually results in unusable and inelegant remnants of fractured droplets.
Another problem encountered by use of known freeze-drying methods is a phenonomen called meltback. Meltback occurs when the heat required during the drying process melts the frozen material. As such, meltback defeats the whole purpose of freeze-drying--the removal of water through sublimation as opposed to evaporation. To avoid meltback in conventional freeze-drying methods, only limited amounts of material of limited thickness can be dried at one time. Even with these limitations, conventional freeze-drying methods are not always rapid enough to prevent meltback.
In the area of pharmaceuticals, known freeze-dried dosage forms do not always exhibit fast dissolution rates when brought into contact with appropriate solvents, such as water, saliva or gastrointestinal fluids. Rapid dissolution of pharmaceutical dosage forms can be of critical importance in instances where it is desirable that the pharmaceutial enter the physiological system as soon as possible.
Thus, there is a need for a method of producing freeze-dried preparations that avoid cracking and meltback during the freeze-drying process. In addition, there is a need for freeze-dried pharmaceutical dosage forms that exhibit rapid dissolution upon ingestion.